1. Field of the Invention
This invention relates to the treatment of gas streams containing sulfur oxides with ammonium sulfite in a manner that minimizes air pollution and produces a useful product while conserving both the ammonia and the sulfur.
2. Discussion of the Prior Art
Sulfur oxides in gaseous effluents are a major source of air pollutants. Such sulfur oxides may be produced by the combination of sulfur-containing fossil fuels or they may result from various processes where sulfur containing compounds are processed. When the sulfur content of the materials being processed is high enough, it is usually recovered either as elemental sulfur or in the form of sulfuric acid.
Many industrial processes and fossil fuel combustion plants, however, produce a gaseous effluent wherein the sulfur dioxide content is a maximum of approximately 2.0% and wherein very small amounts of sulfur trioxide are present. It is with gases of this type that the present invention finds particular utility.
Prior workers in the art have developed numerous types of processes to remove these sulfur oxides. At the present state of development, the most popular process in use employs lime as the active ingredient and produces a throw away calcium sulfitesulfate mixture. Other processes that have been designed with some success employ ammonia and the present invention is concerned with an improvement over such processes. Historically, ammonia scrubbing processes have either processed the scrubber liquid to solids to recover the ammonia and sulfur for fertilizers or other industrial uses, or have decomposed the ammonium salts to liberate the ammonia for reuse and the sulfur dioxide for sulfuric acid production.
The Tennessee Valley Authority has investigated ammonia scrubbing at great length. Their publication "Sulfur Oxide Removal From Power Plant Stack Gas, Ammonia Scrubbing Conceptual Design Cost Study Series", Report APTD-0615, PB 196 804, Sept. 1970 contains both process and cost data on ammonia scrubbing and the production of ammonium sulfate and its use as an intermediate in phosphate fertilizer manufacture. Three processes for treating the scrubber liquid are described. TVA publication "Pilot-plant Study of an Ammonia Absorption-Ammonium Bisulfate Regeneration Process, Topical Report Phases 1 & 2", June 1974, PB-237-170 describes extensively the treatment of scrubber liquor with ammonium bisulfate to produce ammonium sulfate and sulfur dioxide.